Typically, transparent electroconductive films to be utilized as transparent electrodes and the like are each formed by application of a transparent electroconductive paint comprising a solvent including a binder and electroconductive fillers dispersed therein. Further, as electroconductive fillers of transparent electroconductive paints, there have been adopted oxide-based fillers such as indium tin oxide (hereinafter called “ITO”, as the case may be), antimony tin oxide (hereinafter called “ATO”, as the case may be), and the like, and there has been recently proposed a method for using nanorods, nanowires and the like made of gold, silver, or the like as described in a patent document 1, for example.
Incidentally, although metal indium acts as an electroconductor as it is, oxidation thereof produces indium oxide, and additionally doping tin thereinto produces an indium tin oxide (ITO) known as a transparent electroconductive material, so that obtainment of indium nanowires having larger aspect ratios will allow for expectation thereof as fillers of various transparent electroconductive paints.
However, metals for the above-described conventional metal nanowires have been limited to noble metals and the like which are susceptible to be reduced in an aqueous solution, and nanowires have not been obtained up to now based on indium which has a relatively strong bonding force to oxygen.
Further, excellent electroconductive properties such as exhibited by nanowires having larger aspect ratios are not obtained by: the above-mentioned metal nanowires such as the above-described gold nanorods and the like having smaller aspect ratios (ratios of lengths to thicknesses) which can be hardly regarded as wires; metal fine particles which are merely concatenated in chain shapes; and the like; so that there have been desired single crystalline nanowires having larger aspect ratios and being desirably free of grain boundaries, with respect to indium as well.
Meanwhile, in a patent document 2, there has been proposed a production method of a thin single crystalline metallic wire, comprising the steps of: twisting a metallic wire into a helical shape at a predetermined twisting speed during heat treatment in a manner to attain a helix turn having an inclination angle of 20° to 58° relative to a longitudinal direction of the helix, thereby applying a uniform plastic deformation to the wire up to an extent exceeding 98%; and cleaning the obtained wire to eliminate polycrystalline metal residues therefrom. This method utilizes a step of twisting one wire in a yielded condition thereof around a longitudinal axis thereof to perform shear plastic deformation at a deformation ratio higher than 98%, thereby forming a filamentary single crystal. It is described in the patent document that, for production of single crystal filaments of nickel, iron, copper, aluminum, indium, or the like, the twisting step is to be performed together with cooling, and cooling at −170° C. to −200° C. is required for indium wires.
However, in the embodiments of the patent document 2, there are simply described production examples having diameters of 1.3 to 4 μm for single crystal wires of tungsten, copper, steel, molybdenum, and iron, including descriptions of “thin single crystalline metallic wires having diameters of 0.01 to 5 μm” in the paragraph of “TECHNICAL FIELD” and “thin filaments having diameters of 0.1 to 5 μm” in the paragraph of “Problem to be solved by the Invention” of the specification of the patent document 2, so that the patent document 2 fails to show any reason that wires having diameters of 1 μm or less, particularly 0.1 μm or less can be produced.
Further, no descriptions are found concerning diameters of indium wires, even in the embodiments. Thus, the patent document 2 fully fails to disclose production of indium wires having diameters of about 0.1 μm, for example.
Moreover, the single crystalline metallic wires obtained by the above-described method are continuous threads of wires, respectively, which are different from wire-shaped fine particles and thus are not obtained in a state having an excellent dispersibility, thereby becoming short of mass-productivity and undesirable as fillers of transparent electroconductive paints.
Patent document 1: JP 2004-238503A2
Patent document 2: JP 2005-506270T2